Objective: The objective of this review is to explore the concept of phase 0, its contribution and also its practical guidance in drug development process. The process of drug development is protracted, complicated and requires lot of money. Phase 0 focuses on the aspect of microdosing, which determines the relation between PK & PD profile of drug; also selection of lead compound. Concept of phase 0 balances the planning of study scale between animal and human being, and creates a new way of defining phase I. Time is not too far when advance techniques and methods will be developed for the phase 0 studies to make it convenient and widely applicable in the design and development of majority of drugs. Conclusion: Although studies are yet to be done for phase 0 trial, it can be recognized that phase 0 trials would provide an opportunity to generate essential human PK and PD data much earlier in a drug development process, which could be a major advantage in design and decision making for further clinical development of an agent.